The plastic package is the workhorse for most of the commercial semiconductor devices. The leadframe manufacturer's job becomes more complex as the lead count increases and the die size decreases. FIG. 1 is a top-down plan view of a single leadframe 10 prior to being separated from a leadframe strip. While on the strip, the leads 12 of leadframe 10 are shorted together for testing and for mechanical stability. These shorting connections are later removed once their function has been completed. Leadframe 10 is typically formed of a conductive copper (or copper alloy) material having a thickness in the range of generally 5-20 mils, depending upon the required ruggedness of the leads of the integrated circuit package.
FIG. 2 is a cross-sectional view across an integrated circuit package incorporating leadframe 10. As illustrated in FIG. 2, an integrated circuit die 14 having bonding pads thereon is centered with respect to leadframe 10, and thin wires 16 are connected between the bonding pads on the die and the ends of each lead 12 of the leadframe 10. The wires may be aluminum, gold, or any other known wiring material. The resulting structure is then encapsulated in a plastic package 18, and the protruding ends of the leadframe 10 are bent as appropriate for the particular application.
It is desirable to have short wires 16 between leads 12 and the die's bonding pads to limit wire sweep (i.e., lateral movement) during the plastic molding operation. With closely spaced leads 12, such wire sweeping may cause shorting together of adjacent wires 16. Minimizing the wire 16 length also reduces any mechanical forces on the wire bonds during wire sweeping.
As fabrication technologies make available submicron geometries, such as 0.35 micron line widths, the die size shrinks. Since the leadframe must have certain mechanical properties, the pitch of the leads is limited to about 8 mils (125 leads per inch), using conventional etching or stamping processes for forming the leadframes. Thus, with a small die having a large number of bonding pads with a pitch of significantly less than 8 mils, it is easily conceivable that the ends of the leadframe must terminate an undesirably large distance away from the die. This will require the wires extending between the die and the ends of the leadframe to be relatively long, giving rise to the wire sweeping problems already discussed.
One particular integrated circuit known to the applicants is approximately 0.350.times.0.350 square inches and requires 160 leads, with 40 leads per side. The resulting lead pitch is about 9 mils. If the die were smaller, the leadframe leads would terminate farther from the die, and wire sweeping problems may occur. Therefore, there is little incentive to make the chip smaller.
What is needed is a technique to avoid the above problems of wire sweeping so the die sizes can shrink and the number of bonding pads on a die can be relatively large.